Electrodeless lamps are used in various applications wherein the longevity of the lamp is a paramount consideration. Such lamps include a sealed translucent envelope containing a gas which can be excited by electromagnetic radiation to generate high intensity white light. The devices receive electromagnetic energy from a microwave signal which is coupled from a standard magnetron microwave source.
A recent application for high-intensity lamps is in the field of projection television systems. These systems require a source of high intensity white light. The white light is separated into the primary red, green and blue colors, each of which is modulated with appropriate red, green and blue (R G B) signals. The modulated red, green and blue images are combined in conventional dichroic mirror structures to produce a composite color image. A projection lens generates an enlarged display image from the magnified composite signal.
Such devices operate for extended periods of time. Conventional projection television systems rely upon either high intensity discharge arc lamps, or CRT devices which are operated at high electron potentials. These devices have a limited operational life, and a consumer may well need to replace these high-intensity light generators several times during the lifetime of the television system.
The electrodeless lamp technology offers the promise of implementing high-intensity light sources with a life expectancy far exceeding the life expectancies of these other prior art light sources. Sufficient light intensity can be generated from a single electrodeless lamp which is used in conjunction with a conventional reflector structure to distribute the light over the aperture of an optical system for producing the red, green and blue images. The optical requirements for projection dictate that the light source must be small, on the order of 5 mm. diameter. A disadvantage of using the electrodeless lamp in this application includes the requirement that they be microwave-excited. The microwave source must generate microwaves having a power level of 100-400 watts, depending on the projector. Sufficient microwave energy must be coupled to the electrodeless lamp where it is converted into radiant white light. The small size of the lamp requires intense electric fields to couple the energy to the lamp. These power levels produce high levels of heat, requiring that the lamp be cooled by a stream of gas, such as compressed air.
The complications associated with exciting an electrodeless lamp with microwave energy include the requirement that a broad-band low reflection coupling be provided between the microwave source and the lamp. Otherwise, the operating frequency tolerances which accompany different microwave sources, such as magnetrons, may produce an unmatched condition which produces microwave reflections which are received in the magnetron. These reflections shift the frequency of the magnetron, producing further losses in efficiency and a corresponding loss of light output.
The present invention is directed to an apparatus which will couple microwave energy from a standard microwave source to an electrodeless lamp with a small reflection coefficient over a bandwidth representing the frequency tolerance of commercial magnetrons.